Ionizing radiation is employed in a wide variety of medical, industrial and agricultural processes. Some typical processes include radiation therapy of biological subjects or matter (e.g. people, animals, blood), alteration of material properties (e.g. polymerization, cross-linking), quality checking (e.g. inspecting welds, measuringthickness of microelectronic wafers), security scanning (e.g. X-ray detectors), food and crop irradiation (e.g. to grade and sort food, maintain desired properties or remove pests), as well as the sterilization of materials in general. Exposure to ionizing radiation can also occur naturally (e.g. radon gas), accidently (e.g. radioactive spill) or as part of the accepted occupational hazards of certain professions (e.g. miners, radiologists, medical technologists, nuclear plant operators, research scientists, pilots and cabin crew). Regardless of how a person or object is exposed to ionizing radiation, there is often a need to verify that exposure has occurred and to quantify that exposure, albeit acute or chronic.
Radiation dosimeters are used to measure an individual's or an object's exposure to ionizing radiation. A dosimeter is a material or device that, when exposed to ionisation radiation, exhibits a quantifiable change in a physical or chemical property of the material or device which can be related to the dose in a given material using appropriate analytical techniques.
A number of materials have been described that exhibit a dose-dependent property change after exposure to ionizing radiation. For example, the Fricke dosimeter, also called ferrous sulphate dosimeter, is based on the oxidation of ferrous ions (Fe2+) to ferric ions (Fe3+) in water or aqueous gels with a corresponding change in paramagnetic or optical properties that may be measured using nuclear magnetic resonance (NMR) relaxation rates or optical techniques. A major limitation of Fricke solution-based and gel-based systems is the continual post-irradiation diffusion of ions resulting in a blurred dose distribution, particularly when these are used for three-dimensional dosimetry.
Certain crystalline materials (e.g. calcium fluoride, lithium fluoride) exhibit thermoluminescence in response to ionizing radiation. As the radiation interacts with the crystal it causes electrons in the crystal's atoms to jump to higher energy states, where they stay trapped due to intentionally introduced impurities in the crystal. Heating the crystal causes the excited electrons to drop back to their ground state, whereupon a photon is released and can be detected. Since the excited electrons tend to drop back to their ground state over time (so-called fading), thermoluminescence materials possesses a limited shelf life after which dosimetric information can no longer be obtained.
Another material suitable for dosimetry is alanine, an amino acid, which forms radicals upon exposure to ionizing radiation (see e.g. US 2005/0061991 A1). The radicals that are produced are stable for at least several hours. This is mainly due to the inhibition of radical-radical recombination in the crystalline structure of the material where the migration of large molecule fragments is largely prevented. The concentration of formed radicals can be measured using an electron paramagnetic resonance (EPR) spectrometer to determine the amount of absorbed ionizing radiation. Although alanine is widely used as a dosimetry material, the detection of its radicals requires the use of cumbersome and expensive equipment.
Further materials that are reported as being suitable for ionizing radiation dosimetry include radiation sensitive, colour changing diacetylene-type compounds such as R—C≡C—C≡C—R′ where R and R′ are substituents groups (see U.S. Pat. No. 8,115,182 B1), mannitol (see Scherz and Griinewald, Kerntechnik, 1970, v. 12(11), pp. 501-503), and acid-sensitive leuco dyes dispersed in a halogen-containing polymer (see U.S. Pat. No. 5,206,118).
The present invention aims to address disadvantages associated with known dosimetric materials and devices such as those described above.